Brief Statement of the Prior Art
One of the difficulties experienced with the polymerization of methyl methacrylate is that the polymerization becomes uncontrollable when the crude polymerizate exceeds a critical polymer solids concentration. For this reason, prior polymerization processes have avoided bulk polymerization and much of the poly(methyl methacrylate) marketed today is produced by suspension polymerization.
Methyl methacrylate is commonly copolymerized with a limited amount of copolymerizable monomers (comonomers) such as methyl acrylate and ethyl acrylate. These comonomers stabilize the polymer, particularly against depolymerization, which can occur when the polymer chains terminate in unsaturated carbon atoms. The latter results from termination by disproportionation and, accordingly, an objective of the prior art has been to effect chain termination by free radical coupling rather than by disproportionation. This is achieved by incorporating in the polymerization zone a limited quantity of a chain transfer agent such as an alkyl mercaptan, e.g., n-dodecyl mercaptan. The chain termination by free radical coupling to an alkyl mercaptan, however, forms a mercaptan free radical which can re-initiate polymerization, resulting in a polymer chain having a mercaptan terminal group.
Poly(methyl methacrylate) is commonly produced by a suspension process because of the difficulties experienced with solvent or bulk polymerization of this monomer. One of the major difficulties limiting the use of mass polymerization is that the polymerization becomes uncontrollable when the polymerizate reaches a gel condition, typically at solids concentrations above a value which commonly is from 30 to about 40 weight percent. Although solvent polymerization could be used to obviate this difficulty, prior investigators have not developed an efficient method or the equipment required for the devolatilization of large quantities of solvent from the crude polymerizate, preferring instead to use suspension or block polymerization or other techniques, all of which lack in efficiency and/or product consistency. When solvent is present in the crude polymerizate it causes severe foaming during devolatilization and this foaming obstructs efficient heat transfer and devolatilization. Further, the polymer readily discolors if contacted with heat transfer surfaces at temperatures in excess of approximately 270 degrees C. These characteristics interfere with efficient heat transfer and have, heretofore, precluded the successful commercialization of continuous, solution polymerization of acrylate esters such as methyl methacrylate.